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In many parts of the world, fleas are a scourge for cats. Fleas can cause severe skin and ear disease in allergic individuals. Fleas carry diseases and parasites such as tapeworms, Bartonellosis (also known as cat scratch fever), and feline infectious anemia. Severe infestations, in which overwhelming numbers of the blood-sucking parasites feed simultaneously, can trigger fatal anemia. Over the years, I have seen fleas cause massive harm to feline health and well-being.

Happily, there are several safe and effective flea treatments. However, cat owners need to be vigilant, because there also are several highly unsafe medications with packaging that mimics the safe products.

I have repeatedly railed against the products made by Hartz, BioSpot, and Sergeant’s over the years, and I recommend that no pet owner ever purchase any product made by these companies. However, cat owners need to be aware of another, more insidious risk: K9 Advantix.

K9 Advantix is especially dangerous because it is made by the same company as Advantage, and is packaged similarly. Advantage is safe for use in cats. K9 Advantix absolutely is not. Unfortunately, it is very easy to mistake K9 Advantix for its safer cousin, and it’s accidentally applied to cats frequently — often with disastrous consequences.

K9 Advantix contains two active ingredients. One, imidacloprid, is also in Advantage. It protects against fleas but not ticks, and is generally safe for use in cats. The second is permethrin, an old-school insecticide that is effective against ticks. It is, for the most part, safe for use in dogs (although I don’t recommend it, because its margin of safety isn’t sufficient for my taste). Permethrin, however, can be deadly to cats.

K9 Advantix was developed so that Bayer could compete against Frontline (made by Merial) with a claim of efficacy against ticks. However, it is my opinion that where ticks are concerned, K9 Advantix is an inferior product.

In cats (and especially sensitive dogs), the permethrin acts as a nerve toxin. Cats exposed to the product through oral exposure or even contact with the skin may suffer symptoms that start with salivation, facial and whisker twitching, and agitation. These can progress to seizures, coma, and death.

A cat I treated several months ago provides a dramatic example. The owner applied what she thought was a tube of Advantage to her cat. Within an hour, its whiskers began to twitch. The owner double-checked the packaging and realized that she had accidentally applied K9 Advantix for a small dog. She immediately bathed her cat to remove the product. Unfortunately, it was too late. The symptoms progressed.

By the time the cat reached my office, she was seizuring violently and continuously. She had bitten her tongue, and large amounts of bloody froth poured out of her mouth. Ominously, her temperature was below normal. This feature of permethrin toxicity (the bath may also have contributed to hypothermia) is a serious challenge to treat.

The cat did not respond fully to muscle relaxants and anti-seizure drugs, so she had to be anesthetized for several hours to control the urgent symptoms and given cautious thermal support. Over the course of the night, we were able to wean her off anesthesia. Her tremors, disorientation, and facial twitching persisted for several days. Fortunately, the cat made a complete recovery.

The owner blamed herself. I disagreed. In my mind, confusing packaging is the cause of these sorts of crises.

Make sure you check the labels carefully. If fleas are your only concern, Advantage is fine. However, if you are concerned about ticks in your cat or dog, I recommend Frontline (or an equivalent generic product containing fipronil).